Solid polymer electrolyte fuel cells are attracting much attention as low-pollution power sources not only for stationary applications but also for vehicle applications. The fuel cells utilize a compressed hydrogen gas, a high-purity hydrogen gas supplied from a liquid hydrogen tank, or a hydrogen-rich gas prepared by reforming an alcohol such as methanol or a hydrocarbon such as gasoline using a reformer. However, it is known that carbon monoxide or impurities in the hydrogen-rich gas act as poison to platinum in an anode catalyst in some low-temperature fuel cells, lowering the output. To reduce carbon monoxide in the hydrogen-rich gas, the reformed gas is subjected to a shift reaction (CO+H2O→CO2+H2), or an oxidation reaction (CO+½O2→CO2). See, for instance, JP 2002-243707 A, JP 2002-216827 A, JP 11-199202 A, JP 9-268001 A, JP 2001-180911 A, JP 7-232901 A and JP 6-140068 A.
However, an exothermic shift reaction elevates a catalyst temperature as going downstream through catalyst beds, resulting in a lowered CO conversion and a deteriorated catalyst. Carbon monoxide converters having catalyst beds with coolant flow paths are disclosed in JP 2002-216827 A, JP 11-199202 A, JP 9-268001 A, JP 2001-180911 A, JP 7-232901 A and JP 6-140068 A. For example, JP 2002-216827 A describes a carbon monoxide converter comprising a catalyst bed with a coolant flow path in contact with the catalyst, a cooling surface area of the coolant flow path being reduced continuously or stepwise in a gas flow direction to appropriately control a temperature distribution in the converter. However, this carbon monoxide converter fails to sufficiently control the temperature under a high load, resulting in deteriorating the catalyst. This carbon monoxide converter thus should be operated under a low load, failing to reduce the amount of a costly catalyst. The carbon monoxide converter described in JP 2001-180911 A comprises a heat exchanger axially connected to the upstream side of a catalyst bed. However, this carbon monoxide converter is also disadvantageous in that the temperature is not sufficiently controlled under a high load, resulting in deteriorating the catalyst.